Heating unit



June 26, 1945.

H. J. DE N. M COLLUM HEATING UNIT 3 Sheets-Sheet 1 Filed F'eb. 10, 1941 a o a o o 0 June 26, 1945. J McCOLLUM 2,379,017

HEATING UNIT $2M L 2 Z W. Z@ 0 w d m fiv 3 W... J 2

1 w. 5 b. m d m J :QTTQT J. a 6 m 6 v m 7 1 .11! 4 I. v 1. mm,

- June 19457 H. .1. DE N. MOCOLLUM 2,379,017

HEATING UNIT Filed Feb. 10, 1941 s Shets-Shef s mm MQM Patented June 26, 1945 UNITED STATES PATENT oFF cc 2,379,017 nna'rnvG UNIT Henry'J. De N. McCollum, Chicago, 11].; Thelma McOollum executrix of said Henry J. De N.

MeCollum, deceased Application February 10. 1941, Serial No. 378,262 20 Claims. (01.126-116) My invention relates generally to heating systems and, more particularly, to an improved internal combustion heating unit for use in connection with heating systems for airplanes and the like.

sents a suggested combustible mixture. supply system;

Fig. 2 shows a cross-section through the heat exchanger of my improved heating unit along the line 2-2 of Fig. 1;

Fig. 3 shows a cross-section through the combustion chamber of my heating unit along the line H ofFiE. 1; v

fuel supply line or the exhaust line near the heater.

When previous heaters were hermetically sealed, a substantial quantity of the combustible mixture would pass into the combustion chamber and ignite. The rapid expansion caused by the ignition of this relatively large quantity of combustible-mixture would cause an abrupt rise in pressure within the unit, which would prevent the flow of combustible mixture into the combustion chamber until the pressure diminished. The

combustion chamber would then fill with combustible mixture which would ignite and repeat the process, resulting in oscillating combustion. After the heating unit warmed up, these oscillations would become stabilized at a frequency corresponding'to the resonance of the heating unit, causing phenomenon known as motor boating." sure in the interior of'the heating unit would subject it to severe strains which would ultimately cause leaks which would make the heater unsafe to operate, both as to fire hazard and the danger of the escape'of toxic gases of combustion into the air passing into the passenger compartment of the plane. Asa result, previous heating units must employ a pressure relief opening which is usually placed in the combustible mixture intake.

It is, therefore, an object of my invention to provide a combustion chamber and heating unit assembly which is silent and stable in operation when it is hermetically sealed from the surrounding air.

Another object is to provide a heating unit which is efficient, compact, iightin weight and easily installed. and which burns a large volume of a combustible mixture of fuel and air in a small space.

A further object is to provide an improved com- Q bustion chamber and heat exchanger which opcrates eflciently at high internal fiuid velocities.

The rapid rise and fall in pres- Fig. 4 shows a cross-section of the combustible mixture inlet of my heating system along the line 4-4 of Fig. 1; g

Fig. 5 is a cross-sectional view of a modified form of my invention;

Fig. 6 is a cross-section through the line 6-4 of Fig. 5;

Fig. 7 is a cross-sectional view of another modi-.

fled form of my improved heating unit: and

Fig. 8 is a diagrammatic representation, partially in cross-section and partially as a broken away view, of a suggested form for a heater incorporating my improved heating unit.

Referring to Fig. l, a blower l0 forces air through a conduit l2, past the heat exchanger, and a conduit i4 supplies air from the outlet of the blower I! to the inlet of a balanced carburetor it which mixes fuel with the air to form a combustible mixture which passes through a conduit .The outer walls of the heating II to the heater; unit are formed by a generally cylindrical combustion chamber housing 20 and a generally cylindricalheat exchanger shell 22. Thecombustion chamber housing at is lined with a ceramic lining 18 to form a combustion chamber 30.. I Y

Combustion mixture is supplied to the heater through a combustible mixture inlet tube 24 which extends a short distance through the closed end of the combustion chamber. The tube 24 has a generally cylindrical central passage 26 passing longitudinally through its center and closed at the end by a plug 32. Peripheral holes 20 through the wall of the tube 24 are provided immediately adjacent the closed end of the combustion chamber 30. These holes 28 are made of as large a total cross-sectional area as is conveniently possible. I have found that providing eight of these holes and spacing them &2 of an inch apart gives a resulting structure which is Other objects will appear from the following description. reference being had to theaccomq w l drawings. in which:

Fig. 1 shows a vertical cross-section of my improved heating unit, and diagrammatically reprethe housing 20 to prevent the tube 24 from exsufiiciently strong and which has desirable characteristics. .The plug 22 and the end of the passage 24 are preferably shaped to provide a smooth passage for combustible mixture. The walls flare gradually outward from the main portion of the passage 2' into the combustion chamber so that the mixture enters the combustion chamber through pendicular to the 'axis of the tube 24, and no 34 around the against athreaded nipple integrally formed in holes a m a direction substantially percracking of hydrocarbon tending into the combustion chamber 38 more than the proper distance. A threaded hexagonal collar 36, threaded over the nipple 40, seats against the opposite side of the flange 34 to hold the inlet tube in position and to provide for its easy removal should cleaning be necessary.

It is important that the combustible mixture inlet be relatively cool to decrease the rate of flame propagation therein and to prevent the fuel passing through it to form carbon deposits which would interfere with the flow of combustible mixture. Therefore, the inlet tube is constructed to extend a short distance only into the combustion chamber. The passage 28 is preferably of a small cross-sectional area and the total cross-sectional area of the holes 28 is preferably somewhat larger so that the velocity of the combustible mixture through the passage 26 is greater than the rate of propagation therein, while the mixture enters the combustion chamber through holes 28 at a some- I what lower velocity. This arrangement prevents the flame from striking back f rom the combustion chamber 36 and at the same-time reduces the velocity of the mixture passing into the combustion chamber so that it is readily ignited and burned.

An electrically operated igniter 52, having an electrically heated resistance element it of hi chrome wire or other suitable material, is pro vided to ignite the combustible mixture in the combustion chamber 30. One terminal of the resistance element til is grounded and the electric circuit is completed through a wire 68 which connects the other terminal with a source of current supply. A thermostatic switch responsive to the temperature of the heating unit may be placed in series with the wire Zation of the igniter when combustion has been established. A manually operable switch may be provided to control the igniter. The resistance element diis preferably placed in a pocket which has an opening to allow combustible mixture to come into contact with the heated element it. The combustible mixture passes into the pocket where it is retained and heated to above its hash rotating with respect to the casing. Heat ex-- changer fins 66 may be pressed over the outer surface of the shell 22 to increase the heat exchange area thereof, and may be provided with a crease 66 which corresponds to and engages the crease 60 to prevent the fins 66 from rotating with respect to the shell 22:. r

The end of the cylinder disposed toward the combustion chamber 3G is provided with a ceramic reigniter plate 10 which seats againstthe metal cup 72 to close the end of the cylinder 60.

it to discontinue the energipoint to cause a flash which ignites the remainder of the combustible mixture in the combustion chamber. If the resistance element were placed directly in the flow or combustible mixture, the amount of current required to operate the igniter would be larger inasmuch as the combustible mixture would tend unduly to cool the element and it would be difilcultto raise a portion of the mixture to a temperature above its flash point.

The heat exchanger assembly comprises a shell 22 which encloses an apertured ribbed or fluted cylinder til, preferably of niclrel plated copper which is constructed to act as a nonresonant chamber. As is shown more clearly in Fig. 2, this cylinder lid has thickened radially extending ribs 62 around its periphery. These ribs 52 are separated by longitudinal'grooves at which are closed on their fourth side by the shell 22 to form longitudinal extending passageways. Small holes lid are drilled through the cylinder at the bottom of the grooves 56 to provide a re stricted communication between these grooves and the interior of the cylinder lid. In the particular construction shown, the outsidediameter of the ribs 52 is in the order cl two inches and the grooves are sixteen in number. In this construction, I have found that desirable operating characteristics are obtained by making the holes approximately .093 inch in diameter and placing them h inch apart. The dimensions of the ripheral holes 28 at a somewhat reduced velccigg',

ignited by The side of the ceramic plate facing the combustion chamber has reigniter nipples it formed thereon. In the center of each of these reigniter nipples is a smallcylindrical hole it. When combustion occurs in the heater, th'e reigniter plate it becomes heated to a high temperature. Should the combustion flame become temporarily extinguished, combustible mixture will accumulate in the holes 76 and be heated by the walls thereof to cause a flash which will reignite the combustible mixture in the combustion chamber.

The adjacent open ends of the heat exchanger shell 22 and the combustion chamber housing K. are provided with flanges "it and respectively, which are held together by screws B2 to retain the heat exchanger to the combustion chamber. Between these flanges it and Eli is an annular metal spider 84' which is provided with inwardly facing projections 86 to hold the ceramic plate in place and tohold the cylinder 5Q against the closed ends of the shell 22. inasmuch as the cylinder 53 is prevented from rotating by the sin gagement of the grooves 823 with the crease 6G in the shell 22 and the spider is kept from rotatin? by the screws 82, the projections 8:) may be designed and arranged so that they do not obstruct the flow of the products of combustion into the grooves A metal plate at closes the end 05 the cylinder 56 which faces the closed end of the shell The metal plate 2&1 held in piece by which rest against the closed end of the sheh. and seat the plate against a shoulder oithe internal surface of the cylinder 59. Ahole is provided to permit the escape of the products of combustion from the heat exchanger. An e2;- haust pipe 96, which is threaded into a nut ti; brazed to the heat exchanger shell 22, conductthe products of combustion from the heating 113112.

In operation combustible mixture passe; through the passage 26 at a velocity above e rate of iiame propagation thereof, through and into the combustion chamber 8G where it the heated resistance element 6 the igniter Q2. The gaseous products of combostion from the burning of this mixture pass frfc 1. the combustion chamber into the passages forms 1 by the grooves 541. The ends of the ribs EC struct the flow of the gases and tend to bi. up any abrupt rise in pressure. The sharp change in velocity as the gases pass from the comma-rt? chamber into the grooves 54, coupled with the frictional resistance of the walls thereof, tend to damp abrupt changes in pressure caused by slots, or by holes of irregular cross section. Oscillations and waves of pressure in the center of the cylinder 50. are absorbed by the loose packing 20 therein.

Inasmuch as the heating unit is non-resonant, the operation of the unit is stabilized and oscillating combustion is prevented. Therefore, no atmospheric pressure relief need be provided and the cylinder I is retained in the shell 22 by three radially extending ribs I, 120 apart, which space the. cylinder I00 from the walls of the shell 22 and rest against the end of the cawhen the heating unit is installed in a long her- I metically sealed conduit.

The heating unit gives excellent thermal periormance. The hot gases of combustion pass through the passageways formed by the grooves 24 at a high velocity. The eifect or this high velocity is to scour the surfaces in contact with these gases free from surface'fllms of gas molecules having a high resistance to the conductance of heat. Inasmuch as each of these passageways has arelatively small cross-sectional area, there is a relatively large area of contact between the hot gases and the walls of the passages. As the shell 22 and the cylinder 50 are made of a material having a high thermal conductivity, and as the ribs 52 are of a relatively large crosssectional area, heat is quickly and efliciently conducted from the gases of combustion to the fins 04 which convey the heat to the air being heated. The nickel plated surfaces of the walls of the grooves I4, and the heat exchanger shell 22 resist corrosion so that the operation of a heater does not cause an insulating film of oxide to form thereon. I have found that the combustion chamber 22 may be approximately V1 of the size generally recommended for combustion chambersv of similar capacity burning a similar combustible mixture.

,It is preferable to operate my heating unit with its axis in a vertical position, so that any liquid fuel which separates from the combustible mixture passing into the heater when the heater is cold will accumulate adjacent the combustible mixture inlet and will enrich the incoming mixture and thus facilitate its ignition. While my heating unit will also operate satisfactorily in a horizontal position, in this case it may be advisable to provide three igniters spaced 120 degrees apart to insure ignition of the combustible mixture.

The embodiment shown in Fig. 5 is a modified form of my invention. The construction'is much the same as that shown in the embodiment shown in Fig. l, and described above, except that an apertured ceramic cylinder I00 is provided in the inlet end of the heat exchanger shell 22, and the hot gases flow past this cylinder before entering the grooves 54 in the fluted cylinder 50. The ceramic cylinder I00 is provided with small holes I02 which aflord a restricted communication between the outside oi the cylinder I00 and the inside thereof. These holes I02 may be of apramic lining 32 of the combustion chamber 30 to prevent the cylinder I00 from sliding longitudinally within the shell 22. The diameter of the cylinder IIIII corresponds to the diameter of the cylinder and the open end ofthe ceramic cylinder I00. rests againstcylinder 50 to hold the two cylinders in place and to provide a substantially continuous non-resonant central chamber which is filled with a loose heat and corromon resistant packing 58 of mineral or metal wool. Other portions of the apparatus correspond to portions or .the apparatus described in connection with Fig. l. and are similarly numbered.

When the ceramic cylinder I00 is used in the inlet of the heat exchanger casing 22, it is advantageous to reduce the size of the combustion chamber inasmuch as, during combustion, the

ceramic cylinder becomes heated to a high temated-from the ceramic cylinder facilitates combustion oi the gases in contact therewith, and promotes eiilcient operation. Ceramic material has excellent heat radiating properties and the heat which is conducted or radiated from the hot gases to the ceramic cylinder is radiated to the shell 22 and conducted to the has it which dissipate the heat in the air to be heated.

The holes I02 in the ceramic cylinder III act similarlyto the holes 50 in the cylinder II in damping abrupt changes in pressure caused by an explosion in the combustion chamber. It is understood that these holes in the walls of the non-resonant chamber need notbe of circular cross section, but may takethe form of squares, slots, or other shaped openings of a size and shape which has a damping eil'ect on abrupt pressure changes. The ribbed cylinder 2. acts to damp explosions in the manner set forth in the disclosure in connection with Figs. 1, 2 and 8.

In the embodiment shown in Fig. 7-, a packing I20 of chips'of ceramic material is provided in the inlet end of the heat exchanger shell 22. These chips are retained in place by apertured plate I22 having'holes I24 therein to allow the products of combustion to pass from the combustion chamber. The ribbed cylinder I. may be retained in place by the packing I22, or may be secured by other suitable means. Inasmuch as this ceramic packing I20 is heated to a high temperature by the products of combustion, the combustion chamber may be smaller than that used in the embodiment shown in liig. l and combustion may continue within the ceramic packing The reigniter nipples may be dispensed with inasmuch as, should combustion be extinguished,

combustible mixture will gather in the interstices of this ceramic material and be ignited to ignite the combustible mixture in the combustion chamher. The irregular channels for the passage of products of combustion provided by the interstices between the fragments of ceramic material I20 tend to break up, damp and destroy an abrupt rise in pressure caused by an explosion within the combustion chamber 20.

The ribbed cylinder 50 acts in the manner disclosed in connection with Figs. 1, 2 and 3 to facilitate the transference of heat from the products of combustion and to prevent oscillating combustion. Other parts of this embodiment of my invention correspond to similar parts shown in Figs. 1, 2 and 3 and are similarly numbered.

Referring to Fig. 8, which illustrates a heater incorporating my improved heating unit, a plurality of longitudinal fins H extend radially from the shell 22 of the heat exchanger andare brazed or welded thereto. A cylindrical casing H2 encloses the entire heating unit. An electrically operated blower lid supplies air under pressure to the inlet of the casing H2. A conduit H6 connects a point adjacent the outlet of the blower lit with the inlet of a balanced carburetor ilil, and a conduit I connects the outlet oi the balanced carburetor lid with the inlet of the combustion chamber 30. An electrically operated igniter s2 is provided to ignite the combustible mixture within the combustion chamber 80. If desired, a thermostatic switch, responsive to the temperature of the heating unit, may be placed in series with the igniter 42 to discontinue the energization of the igniter when the temper ature of the heater rises to near its operating value. A. conduit E22 connects the outlet of the casing H2 with the space to be heated. Other portions of the heater correspond to similar portions of the heater shown in Fig. 1 and described 'i "i nection therewith, and are similarly numoereo.

.In operation, the blower He draws air from the atmosphere and compresses it. A small portion of this air flows through the conduit M6 to the balanced carburetor lie where it is mixed withiuel to form a combustible mixture, and the combustible mixture passes through the conduit lid to the combustion chamber where it is burned. The gases of combustion pass from the combustion chamber through the heat exchanger, heating the fins lit, and aredischarged through the exhaust pipe 9 3. The hull: of the air com pressed by the blower passes into the casing H2, past the fins iii where it is heated, and through the conduit iii? to the space to be heated.

- providing a fuel and air, means for supplying combustible mixture to said combustion chamber, a heat exchanger receiving the gases of combustion therefrom and having an outlet for discharging said gases, a shell of heat conducting material forming the casing for said heat exchanger, means non-resonant, sound-inhibiting chamber within said shell and disposed to provide space for the passage of said gases of combustion between said chamber and said shell, walls enclosing said chamber, and a plurality of restricted passages through said walls affording acoustic communication between said space and the interior of said chamber.

2. In an internal combustion heating unit, the combination of means providing a combustion chamber for burning a combustible mixture of While a particular construction of the nonresonant chamber has been disclosed, it is understood that a chamber may be given non-resonant properties by the use of bellies to break up sound waves or pressure waves within the chamber, or by the use oi any material within the chamber which will tend to absorb these waves. It is also understood that the provision of restricted openings of a size and shape which break up sound waves or pressure waves passing into or out of the chamber will cause the chamber to have substantially non-resonant properties when used for the purposes of my invention.

While I have disclosed my invention in connection with particular embodiments thereof, it will be apparent to those skilled in the art that numerous variations therefrom may be made without departing from the underlying principles of my invention, and I therefore wish to include within the following claims-all such modifications as may be apparent to those skilled in the art by which substantially the results of my invention can be attained by substantially the same or equivalent means.

Iclaimz- 1. In an internal combustion heating unit, the

combination of means providing a combustion chamber lor burning a combustible mixture of fuel and air, means for supplying combustible mixture to said combustion chamber, and a heat exchanger receiving the gases of combustion therefrom, having an outlet, and comprising 9. metal shell, means providing a non-resonant, sound-inhibiting chamber therein shaped generally to correspond to the outline thereof, and so disposed with relation to said shell as to provide space between the walls of said shell and the walls of said chamber for the passage of the products of combustion from said combustion chamber to said outlet, and a plurality of restricted passages affording communication between said space and the interior of said cham. ber whereby sound may enter said chamber and be absorbed therein.

3. In an internal combustion heating unit the combination of means providing a combustion chamber for burning a combustible mixture of fuel and air, means for supplying said combustible mixture to said combustion chamber, a heat exchanger for receiving the gases of combustion from said combustion chamber, having an outlet and comprising a shell of heat conducting material, means providing a sound-inhibiting chamber of heat conducting material within said shell and shaped to correspond in outline thereto, a pinrality of ribs of heat conducting material between and engaging said shell and said chamber and disposed in the direction of flow of gases through said heat exchanger for conducting heat Zrom said chamber and said gases to said shell, and having spaces between them for the passage of combustible mixture from said combustion chamber to said outlet, and sound-absorbing material in said chamber, said last-named means having openings connecting said spaces with the interior of the chamber formed in said means, said openings forming acoustic couplings between said spaces and said material.

4. In an internal combustion heating unit, the combination of means providing a combustion chamber for burning a combustible mixture ofiuel and air, means for supplying combustible mixture to said combustion chamber, and a heat exchanger for receiving the gases oi combustion therefrom, having an outlet, and comprising a shell of heat conducting material, means providing a closed sound-inhibiting chamber of heat conducting material within said shell, having walls disposed generally to conform m the outline of said shell, and provided with ribs extending longitudinally along the outside thereof and engaging the internal surface of said shell, said ribs having spaces between them providing passages for conducting said gases or combustion from said combustion chamber to said outlet, and a plurality of restricted openings in the walls of said closed chamber affording communication between said passages and the interior of said cylinder'.

5. An internal combustion heating unit having means providing a combustion chamber for burning a combustible mixture of fuel and air, means for supplying combustible mixture to said combustion chamber, a heat exchanger for receiving the gases of combustion therefrom, and comprising a metal shell open at one end to receive said gases from said combustion chamber and having an outlet for discharging said-gases, means providing a non-resonant, sound-inhibiting chamber having walls of ceramic material within said shell adjacent said open end and so disposed with rerespect to said shell as to provide space for the passage of said gases between said walls and said shell, means providing a second non-resonant chamber having metal walls disposed generally to conform to the outline of said shell located within said shell between said first-mentioned chamber and said outlet, a plurality of longitudinally extending ribs engaging said metal walls and the inside of said shell and having spaces between them to provide passages for the flow of said gases between said metal cylinder and said metal shell, and a plurality of restricted openings in said walls afiording communication between said passages and the interiors thereof.

6. An internal combustion heating unit comprising means providing a combustion chamber for burning a combustible mixture of fuel and air therein, means for supplying combustible mixture to said combustion chamber, a heat exchanger receiving the gases of combustion from said chamberand comprising a metal shell open at one end to receive said gases of combustion and having an outlet at the other end, a packing of ceramic fragments filling that part of said shell adjacent said open end, means for retaining said packing in place, means providing a non-resonant, sound-inhibiting chamber having metal walls within said shell adjacent said outlet, a plurality of ribs engaging the outside of said metal walls and the inside of saidshelland having spaces between them to provide passages for the flow of products of combustion from said packing to said outlet, whereby said ribs conduct heat from said metal wallsv and said products of combustion to said shell, and a plurality-of restricted openings in said walls affording communication between said passages and the interior of said non-resonant chamber.

7. In an internal combustion heating unit,- the combination of means providing a combustion chamber burning a combustible mixture of liquid fuel and air, a dead end inlet tube extending a having an outlet for discharging said gases of 'combustion.

8. In an internal combustion heating unit the combination of means providing a combustion chamber burning a combustible mixture of fuel andiair therein, means for supplying combustible mixture to said combustion chamber, a heat exchanger receiving the gases of combustion from said chamber and comprising a shell of heat conducting material having an opening to receive the gases of combustion from said combustion chamber, an outlet for discharging the gases of combustion therefrom, means providing a nonresonant, sound-inhibiting chamber within said shell, shaped generally to conform to the outline thereof and so disposed with relation to said shell as to provide space between said chamber and said shell for the passage of the gases of combustion from said combustion chamber to said outlet, metal walls enclosing that portion of said non-resonant chamber adjacent said outlet, a plurality of ribs ailixed to the outside of said walls and disposed in the general direction of flow of gases of combustion from said chamber to said outlet, said ribs engaging the inner surface of said shell to space said walls-from said shell and to provide passages to said gases of combustion between said ribs, walls of a ceramic material enclosing that portion of said chamber adjacent said combustion chamber and spaced from'the walls of said shell to provide for the passage of ases of combustion between said walls and said shell, and a plurality of restricted openings in said walls ailording communication between the incombustion chamber, a heat exchanger receiving the products of combustion from said combustion chamber and having an outlet, said heat exchanger comprising a shell of heat conducting material, means provldinga non-resonant, soundinhibiting chamber within said shell, walls of-heat short distance into said chamber for supplying combustible mixture thereto, a central passage through said tube for conducting combustible mixture at a velocity abovethe rate of flame propagation thereof, and h0les of greater total cross-sectional area than said passage around the periphery of that portion of said tube extending into said chamber to conductsaid mixtune from said passage into said chamber, whereby said mixturev enters said chamber in a direction substantially perpendicular to the axis of said tube, and a heat exchanger, having an inlet for receiving the gases of combustion fromsaid combustion chamber, having a non-resonant chamber therein in communication with the gases of combustion from said combustion chamber, and

conducting material enclosing that portion of said non-resonant chamber adjacent said outlet and shaped generally to conform to the outline of said shell, longitudinally extending ribs onthe outer surface of said walls, said ribs being disposed in the direction of flow of gases of combustion through said heat exchanger and engaging the inner surface of said shell to form passages between said ribs for the flow of said gases of combustion, walls of ceramic material enclosing that chamber affording communication between the interior of said chamber and said passage, and a .loose acking of a heat and corrosion resistant changer, said heat exchanger cooperating with 11. An internal combustion heating unit comprising means providing a combustion chamber for burning a combustible mixture of fuel and air therein, means for supplying said mixture to said combustion chamber, a heat exchanger adjacent said combustion chamber for receiving the gases of combustion therefrom, and an outlet for discharging said gases of combustion from said heat exchanger, said heat exchanger comprising a shell of heat conducting material, a non-resonant chamber within said shell and shaped generally to conform to the outline of said shell, walls of heat resistant material enclosing said nonresonant chamber and so disposed with respect to said shell as to provide space for the passage of said gases of combustion between said walls and said shell, a plurality of restricted openings through said walls affording communication between the inside and outside thereof, and a plurality of nipples of ceramic material, each having a small open cavity, aflixed to that portion of said walls adjacent said combustion chamber.

12. A heater comprising a hermetically sealed casing, an inlet conduit for conveying a combustible mixture of fuel and air thereto, a discharge conduit for conveying the products of combustion therefrom, means for maintaining a substantial pressure difference between said inlet and outlet conduits to cause rapid flow through said casing, a combustion chamber at the inlet end of said casing, electrical means to ignite the fuel mixture in said combustion chamber, an enclosure in said casing forming a compartment occupying the major portion of the space within said casing and providing longitudinally extending passageways between it and the inner wali of said casing, said enclosure having a plurality of spaced apertures connecting said passageways with said compartment, and an inert air vibration damping material substantially filling said compartment.

13. The combination set forth in claim 12 in which said enclosure is made of copper plated with nickel.

14. The combination set forth in claim 12 in which said damping material is stainless steel wool.

15. The combination setforth in claim 12 in which said enclosure is formed in part by a hollow ceramic cup-shaped element with its closed end forming one wall of said combustionchamber and in part by a ribbed hollow metal cylinder, the ribs of which form said passageways and are in good heat conducting contact with said casing.

16. The combination set forth in claim l2 in which said casing is surrounded by heat conducting longitudinally extending fins enclosed in a housing. and in which the means for maintaining the substantial pressure difference between the inlet andoutlet conduits causes flow of air to be heated through the space between said casing and said housing.

17. A heating unit for burning a combustible mixture of fuel and air, comprising means proother, said holesopening into said passage and forming acoustic couplings between said passage and the interior of said last-named chamber, and means for removing the products of combustion from said heat exchanger.

18. In a heating unit generating heat by the burning of a combustion mixture of fuel and air therein, the combination of means providing a combustion chamber for burning said combustible mixture, a heat exchanger receiving the products of combustion therefrom and having an exhaust outlet, said combustion chamber and heat exchanger forming a sealed path leading to said exhaust outlet, means providing a sound-inhibiting chamber in said path and a plurality of restricted communications between said path and the interior of said chamber, and means defining longitudinally extending passageways conducting the products of combustion from said combustion chamber, through said heat exchanger, to said outlet, said means being in heat conductive relationship with the walls of said heat exchanger.

19. In a heating unit generating heat by the burning of a combustible mixture of fuel and air therein, the combination of means providing a combustion chamber for burning said combustible mixture, a carburetor having an air inlet and connected to said combustion chamber for supplying a combustible mixture thereto, a heat exchanger receiving the products of combustion from said combustion chamber and having an exhaust outlet, said combustion chamber and heat exchanger forming a path leading to said exhaust outlet and conducting products of combustion thereto, said inlet and outlet being adapted for exposure to different pressures toproduce flow through said combustion chamber and heat exchanger, and means providing a sound-inhibiting chamber in said path, said last-named means having a plurality of restricted communications between said path and the interior of said soundinhibiting chamber and forming acoustic couplings therebetween.

20. In an internal combustion heating unit, the combination of means providing a combustion chamber burning a combustible mixtureof fuel and air, an inlet tube connected to said chamber for supplying combustible mixture thereto, a heat exchanger receiving the products of combustion from said combustion chamber and having an cx-v haust outlet, said combustion chamber and heat exchanger forming a path leading to said exhaust outlet and conducting products of combustion thereto, means providing a sound-inhibiting chamber in said path, said last-named means atfording a plurality of restricted openings between said path and the interior of said sound-inhibiting chamber, and sound-absorbing material in said sound-inhibiting chamber for absorbing sound entering said last-named chamber through said restricted openings.

HENRY J. DE N. MCCOLLUM. 

